In the field of electronics, it is quite common to secure a component such as an integrated circuit (IC), to a substrate such as a printed circuit board (PCB). In a typical application, the IC is in a package with leads along two or more edges. These leads are connected to a PCB in a variety or ways, typically by soldering. With larger components with higher lead counts, many packages have leads along all four edges.
If such a component is stressed by applying a force on the component orthogonal to the plane of the substrate, if the force is applied in the vicinity of the leads, the leads provide good mechanical resistance to the force (within limits, of course, beyond which the leads may fail and collapse). However, if the force is applied to a region of the component which is not close to the leads, there may be a considerable lever arm between the region of the component which is supported (near the leads) and the point at which the force is applied. These principles apply equally well to a force which has a component orthogonal to the plane of the substrate. Referring to FIG. 1, if the span between support points (e.g. leads 10) is significant, a component 11 may be displaced a significant amount when resisting an external force 12 such as a shock.
A traditional package for an IC may be mechanically rigid and transfer force efficiently from where it is applied to a fulcrum, such as the leads. Some newer packaging, particularly C4-QFP (controlled collapse chip carrier-quad flat package) packages, are not as rigid mechanically. In fact, for a PowerPC processor (Motorola, Phoenix, Ariz., or IBM, Essex Junction, Vt.) such as a PowerPC 601 or PowerPC 604, the package dimensions are approximately 1.8 inches by 1.8 inches between edges and leads. Referring to FIG. 2, package 21 is secured to PCB 22 by leads 23. Integrated circuit die 24 is affixed to the top of package 21. Heatsink 25 is positioned firmly over die 24 by holder 26, which is secured in turn by clips 27 which are pushed through and lock under holes 28 in PCB 22. Die 24 and heatsink 25 preferably are thermally connected by a thermal grease such as TC208 (available from Thermoset) or TC228 (especially for PowerPC 604) or the like. This helps conduct heat from die 24 to heatsink 25.
In a typical manufacturing process, a component such as a PowerPC IC is positioned on and secured to a circuit board. In a subsequent step, a heatsink is positioned over the component, then secured by a holder. During the positioning and securing of the heatsink, it is very easy to apply a force at or near the center of the component, orthogonal to the PCB, and bend the component. In some packages, especially C4 QFP, the die is exposed and has no mechanical protection against deformation other than that provided by the component package. In a significant number of instances, the deformation exceeds allowable limits and the die cracks, rending the component non-functional. This, in turn, makes the PCB useless and the component must be replaced. In a typical manufacturing operation, well over 20% of the board failures can be traced to a cracked die. Furthermore, once a board is shipped, the heat sink is a prominent feature on the PCB and can be accidentally or deliberately engaged. In some instances, inexperienced personnel pick up a PCB only by the heat sink, thus applying a significant force as the board tends to rotate at an angle to the heat sink, applying a potentially damaging force to the IC and the die. A substantial portion of field failures of the board are also due to cracked die.
A component such as a PowerPC IC may also be damaged through inadvertently pressing on the die through other actions. Die cracking was observed as a result of poor handling practices and inability to follow rigid guidelines for attaching and handling heatsinks. Some specific problems were:
1. Twisting or flexing the PCB. PA1 2. Stacking populated boards. PA1 3. Carelessness when placing assembled boards in totes or boxes. PA1 4. Resting the top side of the board on a hard surface damages microprocessor die. PA1 5. Using too much pressure to apply heatsink. PA1 6. Using uneven pressure to apply heatsink. PA1 7. Pushing down on a correctly attached heatsink. PA1 8. Not using the correct heatsink placement tool. PA1 9. Leaving heatsink loose by not engaging all four heatsink clips into the board. PA1 10. Using the heatsink as a handle to pick up and move the populated board.
Prior to this invention, there has not been a useful solution to this problem. Over 1.3 million PowerPC computers have been shipped by Apple Computer alone and more PowerPC computers have been shipped by other vendors. This potential problem is common to many if not all, of these computers.
This type of problem may exist for other components as well. In general, this is less of a problem where the span between leads is relatively small. Also, the problem is minimized where the packaging is particularly rigid. However, whenever a component includes a brittle sub-component, such as a silicon die, that may flex beyond tolerable limits under certain conditions, the present invention may provide a benefit. The problem is likely to be exacerbated for a component with widely spaced leads (e.g. edge-to-edge across a component) with no central or intermediate support.